1. Field of the Invention
This invention relates to a process for treating bauxite to make high purity alumina.
2. Prior Art
Bauxites are reacted with caustic soda solution at elevated temperatures to dissolve the alumina and, after cooling, the resultant slurry is separated into insoluble matter and a pregnant liquor from which alumina hydrate can be precipitated. The alumina hydrate will contain some iron oxide and, if this is above 0.017% Fe.sub.2 O.sub.3, its value is decreased, as it is difficult to make high purity aluminum from the alumina hydrate.
The pregnant liquor, resulting from the reaction of bauxite and caustic soda, contains iron oxide in the form of ferric and ferrous iron, which will both co-precipitate with the alumina hydrate. The higher the digestion temperature, the greater is the amount of ferrous iron solubilized from the bauxite. The ferric iron is easily removed, using simple filtrating techniques, such as filtering the pregnant liquor through paper, sand or a synthetic cloth, whereas the ferrous iron, being in a colloidal or soluble state, is difficult to filter out unless sophisticated techniques are used, such as filtering the pregnant liquor through a bed of sand, iron grit (see U.S. Pat. No. 3,792,542) or bauxite (see U.S. Pat. No. 3,728,432) where the ferrous iron adheres to those particles due to van Der Waals forces. The disadvantages of these methods using beds of iron grit or bauxite are the high capital cost of this type of filter, difficulty in operating and increased operating cost attributable to purchasing and installing the bed.
Filtration techniques, using membranes of synthetic cloth, are simpler to operate and are used widely in the industry. In the laboratory, membranes of caustic resistant paper are used and can duplicate industrial filtration, using synthetic cloths.
Bauxites, which originate from such areas as the Darling Ranges and the Gove Peninsula in Australia or the Trombetas area of Brazil, need only be reacted with caustic soda solutions at temperatures from 113.degree. C. to 205.degree. C. so little ferrous iron is formed and the alumina hydrate generally contains less than 0.017% Fe.sub.2 O.sub.3.
Bauxites, which originate from such areas as Weipa in Australia or Boke in West Africa, are reacted with spent caustic soda solutions, generally at more elevated temperatures ranging from 206.degree. C. to 350.degree. C., so a greater amount of ferrous iron is formed and the alumina hydrate can contain more than 0.017% Fe.sub.2 O.sub.3 unless sophisticated techniques are used to remove all or part of the ferrous iron.
In another method (see U.S. Pat. No. 3,607,140), the iron impurities in the liquor are co-precipitated with a small amount of alumina hydrate. However, this is a sophisticated techique and consumes additional energy to redigest the alumina hydrate precipitated with the iron impurities.
In U.S. Pat. No. 4,324,769, a double digestion process is shown which lowered the iron oxide in alumina to less than 0.03% if in the second digest the mud was mixed with bauxite in the ratio of 0.1 to 2.5 times the weight of bauxite. It has now been found that this patent can be improved and is the subject of this application. To produce alumina hydrate economically and to be able to sell it, an alumina process must meet all the following constraints--low energy usage, minimum capital cost, high extraction efficiency and low iron oxide in the alumina hydrate. Other prior art either does not meet these constraints or are unlike applicant's process of this invention.
A process described in the publication entitled "Extraction of Alumina from Ferruginous Bauxite by a Double-Leech Process" by W. F. Holbrook and L. A. Yerkes and published by the U.S. Department of Interior is designed primarily for bauxites containing more than ten percent silica and entails calcining the bauxite, removing the reactive silica by leeching with ten percent caustic soda at 90.degree. C., separating the solids from the liquor, regenerating the caustic in this liquor with lime and recirculating the liquor back to the leeching zone.
Another prior art process is discussed in Scondrett, U.S. Pat. No. 2,852,343. Two different bauxites are digested in two parallel digestive units. One bauxite is predominately trihydrate; the other is a mixture of trihydrate and monohydrate alumina. This process is unlike applicant's process of this invention.
Another prior art process is discussed in Porter, U.S. Pat. No. 2,701,751, entitled "Process for Production of Alumina" and issued February 8, 1955. The process disclosed requires the liquor obtained from the second digester unit at an Al.sub.2 O.sub.3 to NaOH weight ratio of 0.42 to 0.54 be recycled to the first digester unit in order to raise its Al.sub.2 O.sub.3 to NaOH weight ratio to 0.8 before it can be routed to precipitation. This recycling of the hot monohydrate alumina extraction liquor from separatory unit 2 back to the first or trihydrate digester unit upsets the heat balance on this first digester unit. That is, the temperature of the first digester effluent after the flash tank will exceed 112.degree. C. because not enough steam can be flashed to the liquor heater and condensed by the hot recycled liquor so excessive heat losses will occur. The process described by Porter may have been acceptable when oil was $3.00 per barrel, but causes excessive heat loss for it to be used today. Porter's patent can be improved and the heat losses reduced by thirty percent by setting the conditions in the second digester unit so that the Al.sub.2 O.sub.3 to NaOH weight ratio can be increased above 0.75 and the monohydrate extraction liquor then routed directly to precipitation. This, however, would not then be Porter's patent but the subject of one improvement made in this application by applicant.
Another prior art process is discussed in Australia Seimiya, et al Pat. No. 269,395 entitled "Process for Extracting Alumina from Bauxite Containing Both Alumina Monohydrate and Trihydrate". This patent discloses an unground bauxite in lump or granular form which is placed in a packed column and contacted with caustic soda at 100.degree. C. to 140.degree. C. to dissolve the trihydrate and leave the monohydrate undissolved. The effluent from this column is separated into sodium aluminate solution, granular residue and muddy substance. The muddy substance is discarded, the lump or granular form residue of generally plus 20 mesh containing some alumina monohydrate is digested in an autoclave digester with fresh alkali solution at a temperature greater than 160.degree. C. The digester effluent is separated into sodium aluminate solution and a red mud which is discarded. The alumina in the sodium aluminate solution is recovered in precipitation.
This process has two serious disadvantages which can be improved as (1) it is impractical to pack a large enough column for industrial use with lumpy or unground bauxite and (2) the muddy substance discarded from the first digest can contain significant amounts of monohydrate alumina and make the process uneconomical and this is indicated in the two examples quoted by the low extraction rate of 89.5% and 90.1%, respectively.
Seimiya's patent can be improved by grinding the bauxite in caustic liquor so it can be pumped in slurry form into digesters to be reacted with caustic soda. The grinding of the bauxite makes a majority of the monohydrate alumina report to the muddy substance and not to the granular residue and the alumina monohydrate can then be extracted from the muddy substance and this is one improvement considered in this application by McDaniels.